JOHNSTON, Iowa — The Republican debate Wednesday belied the fierce, increasingly negative battle in Iowa and elsewhere for the party’s presidential nomination.

The candidates largely held their fire in their final meeting before the state’s Jan. 3 leadoff caucuses, mindful that negative campaigning can backfire, particularly in Iowa.

“Let’s raise the level of dialogue and discussion and debate in this campaign,” Arizona Sen. John McCain proposed, well into a 90-minute debate remarkably free of acrimony.

Offstage, the GOP race has turned sharply contentious.

In Iowa, religion and immigration are dominating the campaign three weeks before the caucuses. With polls showing a dead heat, Mitt Romney and Mike Huckabee have squared off daily on both issues.

Romney, a former Massachusetts governor, started running a TV ad this week — the race’s first negative commercial — accusing Huckabee, a former Arkansas governor, of being soft on illegal immigration. Huckabee, for his part, invoked Romney’s Mormon faith in an interview. The ordained Southern Baptist preacher asked: “Don’t Mormons believe that Jesus and the devil are brothers?”

But none of that was raised in the debate, and the nine men on the stage were on their best behavior. Most passed up multiple opportunities to go after each other, instead restating their positions on everything from trade to taxes.

The subject of education produced the only semblance of sparks.

Moments after Huckabee said schools should provide all students with music and art instruction at all grade levels, Colorado Rep. Tom Tancredo took him to task. “That’s not the job of a president. It’s the job of a governor,” he said. “That’s what you should run for if you want to dictate curriculum.”

Huckabee responded by saying that in his decade as governor he had the “most impressive education record.”

That brought a polite disagreement from Romney. “I just wanted a small adjustment to what Governor Huckabee had to say. And I don’t believe you had the finest record of any governor in America on education,” he said, eliciting laughter from the debate audience.

Indeed, Arkansas public school students scored at or below the national average in four federal education test categories in 2007; Massachusetts public school students scored first in all areas, math and reading for both fourth- and eighth-grades.

McCain, assailed in corn-growing Iowa for his opposition to ethanol subsides, all but called his rivals fakes, saying: “I don’t believe that anybody can stand here and say that they’re a fiscal conservative and yet support subsidies” that hurt the U.S. economy.

Carolyn Washburn, editor of The Des Moines Register and the debate moderator, brought about a mini-revolt at one point when she asked the candidates to raise their hands if they thought global warming was a serious threat caused by human behavior. “I’m not doing hand shows today,” said Thompson. “You want to give me a minute to answer that?”

“No, I don’t,” Washburn said.

“Well, then I’m not going to answer it,” Thompson said as former New York Mayor Rudy Giuliani, McCain and others talked over one another.

Ultimately, no one disputed global warming was a problem or that humans at least contribute to it.